The invention is concerned with a device for the digital measurement of the rotational speed of a rotating member, for example of a vehicle wheel, in particular for antiskid control systems, having a sensor which produces a sequence of signals proportional to the rotational speed, and having a measurement device, which is provided with a counter and a pulse generator, for counting the counting pulses of a certain frequency which are supplied by the pulse generator and which fall within the duration of a cycle or of a time interval which is dependent upon the cycle duration.
In systems employing digital measurement by means of sensing a rotating wheel with a sensor and the measurement of the cycle duration of the sensor signal by counting high-frequency pulses falling within the duration of the sensor signal cycle, essentially two related problems occur:
1. If not too high a counting rate is chosen, there occur great errors in quantization in the upper speed range, as determined by the relatively large time range of the cycle duration assigned to one pulse of the pulse train, i.e. by a relatively large quantization unit.
2. If the counting rate is increased in order to keep the error in quantization in the upper speed range small, then very high counting results are obtained in the lower speed range, which requires the use of multi-digit counters and means a substantial increase in the data word length, thus causing a very considerable increase in circuit-related and component-related expenditures, in particular with regard to the evaluation.
Consequently, it has been already suggested to not only limit -- in the direction of higher speeds -- the respective measuring time to the duration of a cycle of the sensor signal but to have several cycles fall within the measuring time and to subsequently correct the counting result accordingly. It is a disadvantage that this measure does not allow the error in quantization to be reduced. There is obtained merely a mean value from several cycles. Speed changes which occur during the relatively long measuring time cannot be detected, so that the sensitivity of such an arrangement is low, at least in the upper speed range, which is particularly important for the brake power regulation.